How to store your starter
When you're not using it, store your sourdough starter in a jam jar in the fridge. This slows it down so it can go a week or two without being fed. Once your starter is established, you are unlikely to ever need more than around 20g at any time so you can put any leftover into a separate 'discard' jar in the fridge.
Sometimes you'll see a layer of liquid on top of your starter or discard. This is called 'hooch' and is a byproduct of fermentation. It's a sign that the happy little yeast and bacteria in your starter have eaten everything they can and they are hungry. Hooch is safe to eat so you can either pour it out or just stir it back in.
Although your starter is fairly robust once it is established, you should still be very careful to only use clean utensils with it and regularly change it into clean glass jars.
Before you want to use it
It's a good idea to feed your starter once or twice before you want to use it, especially if it has been sitting in the fridge for a bit. Feeding it wakes it up, keeps it strong and makes it more likely to work properly when you need it. Strong starter = better bread.
For an interim feed like this, take your starter out of the fridge and discard most of it into your discard jar.
Mix a ratio of 1 part starter to 5 parts water and 5 parts flour (eg 10g starter + 50g water + 50 g flour) in your starter jar and leave it on the kitchen counter. If it doubles in volume and looks bubbly within around 4-8 hours, then it's already pretty strong and will be ready to start baking with. If it's a bit sluggish, then you'll need to feed it again daily, using the same technique and keeping it out of the fridge, until it has gotten stronger.
As you get more confident and familiar with your starter, you can play around with the ratios of starter to water and flour to change its strength and activity:
How to stop getting overwhelmed by large volumes of starter and discard
The internet is full of huge jars of starter and recommendations to feed it twice a day. This is painful and a waste of flour. You don't need to do this.
only keep around 10-20g of starter between feeds
store it in the fridge so you only need to feed it when you are going to be baking soon
if you're feeding it to get it strong/wake it up, just use 5g of starter and a 1:5:5 ratio. This will create 55g of new starter. Discard the spare old starter, and 35g (ie 55g-20g) of new starter so you have 20g starter remaining.
if you're feeding it ready for baking, use a 1:10:10 ratio with 10g of starter. This will get you to 110g, meaning that you can use 100g starter in your recipe and keep 10g in a new starter jar for next time
Troubleshooting
Starter won't rise
How do I know when it's ready to bake with?
What to do with sourdough discard
There are lots of recipes for using up sourdough discard so you can just let it collect in the discard jar in the fridge until you have enough to use. Discard gets more sour over time and gives a nice flavour to things like pancakes and crackers.
Simple sourdough discard pancakes (makes 2-3 pancakes, enough for 1 person)
125g discard
1 egg
1 tsp butter
1/8 to 1/4 tsp bicarbonate of soda (higher amount makes pancakes fluffier)
Extra butter for cooking pancakes
Stir together sourdough discard and egg in a bowl. Melt butter in pan on medium high heat, then stir into mixture. Stir in bicarb of soda and wait 2-5 minutes for mixture to get bubbly. Pour batter into buttered pan and cook on both sides until done. Top with sweet or savoury toppings as desired.